1. Field of the Invention
The present invention relates to current sense devices, and more particularly to a rail to rail current sense amplifier.
2. Description of the Related Art
Portable devices, including laptop computers and the like, include a rechargeable battery that provides power to the device when AC power is not available. The battery is typically removable, and is either not connected or is connected and receiving charge current from battery charging circuitry when AC power is available. The charging circuitry includes a current sense circuit which monitors charge current so that the battery charging process may be controlled. The current sense circuit usually includes a current sense device, such as a small-valued current sense resistor or the like, which is coupled in the charge path of the battery and a current sense amplifier coupled to sense voltage across the current sense device, which voltage is proportional to the charge current.
In conventional designs, the current sense device is located at the positive terminal of the battery and the sense amplifier includes power rails with a voltage level that is typically significantly smaller than the voltage of the battery. The current sense circuit, therefore, performs two functions, including level shifting to within the voltage range of the sense amplifier, and gain to amplify the sense voltage by a suitable amount. The gain function places the voltage range within appropriate voltage ranges for a variety of conditions, including low battery voltage and high charge current, and high battery voltage and low (e.g., trickle) charge current.
It is readily apparent that the relatively small voltage signal across the current sense device is attenuated by a relatively large factor down to an even smaller voltage level. Such attenuation was otherwise necessary to attenuate the higher battery voltage levels to the lower voltage levels suitable for the sense amplifier. The sense amplifier required a relatively high gain, such as on the order of about 100 or so, in order to place the output sense voltage in a suitable voltage range for monitoring circuitry. In this configuration, the inherent voltage offsets of the sense amplifier became a proportionately larger factor in the measurement range resulting in relatively inaccurate charge current measurement. It is desired to avoid significant attenuation of the measurement signal while also enabling the use of lower voltage measurement devices.